The present invention relates to a method for producing a modified polyester having high fiber whiteness and excellent dyeability.
The well-known method for producing polyesters having superior dyeability comprises copolymerizing ethylene terephthalate with a polyalkylene glycol (Japanese Patent Publication No. 34-10794). However, the polyester manufactured by this method had the disadvantage of poor fiber whiteness.
There was accordingly proposed a method for producing a polyester of high fiber whiteness which comprises subjecting terephthalic acid and ethylene glycol to esterification and polycondensation reactions, with a polyalkylene glycol being added to the reaction system for copolymerization at the stage when the degree of esterification has reached 70% (Japanese Laid-Open Patent Publication KOKAI No. 52-63292).
However, in this method for producing polyesters, it is essential that one should start with terephthalic acid and ethylene glycol and, then, add a polyalkylene glycol at the stage when the degree of esterification has reached at least 70 percent. This requirement caused the following problems.
(1) Process control and control of production conditions are difficult.
(2) The sequence of transesterification reaction between dimethyl terephthalate and ethylene glycol and subsequent condensation reaction does not have any favorable effect on improving fiber whiteness, and furthermore affects spinning tone adversely.
In addition, no satisfactory degree of whiteness can be accomplished by the method.
Therefore, the present inventors conducted a series of experiments and found that salts of potassium and sodium occurring as impurities in the polyalkylene glycol charge exert marked deleterious effects on the whiteness of the product polyester fiber. The present invention is predicated on the above finding.